/Engine/Source/Runtime/Core/Public/Containers/SparseArray.h
Allocates space for an element in the array at a given index. The element is not initialized, and you must use the corresponding placement new operator to construct the element in the allocated memory.
[FSparseArrayAllocationInfo](API\Runtime\Core\Containers\FSparseArrayAllocationInfo) InsertUninitialized ( int32 Index )